You can change the SQL server that the GPA server is pointing to by preforming an uninstall of the GPA server and installing the server piece again. During the install of the new server you need to point to the GPO_REPOSITORY on the new SQL server.
You can also change the SQL server by using the below procedure.
1) Make sure that both SQL servers are online
2) The GPO_REPOSITORY db should be attached in both SQL servers
3) Execute the NQGPARepConfig.exe tool with /disconnect option (i.e. > NQGPARepConfig.exe /disconnect /Repository:) located in the tools fold in the GPA installation directory.
4) In GPA Server Configuration GUI enter the new SQL server name and press Apply